Overview of IAM Policies

This section provides an overview of IAM policies. A policy is a document that formally states one or more permissions.

For a complete reference to the IAM policy syntax and grammar, see AWS IAM Policy Reference.

Introduction

To assign permissions to a user, group, role, or resource, you create a policy, which is a document that explicitly lists permissions. In its most basic sense, a policy lets you specify the following:

Policies are documents that are created using JSON. A policy consists of one or more statements, each of which describes one set of permissions. Here's an example of a simple policy.

{
  "Version": "2012-10-17",
  "Statement": {
    "Effect": "Allow",
    "Action": "s3:ListBucket",
    "Resource": "arn:aws:s3:::example_bucket"
  }
}

You can attach this policy to an IAM user or group. If that's the only policy for the user or group, the user or group is allowed to perform only this one action (ListBucket) on one Amazon S3 bucket (example_bucket).

To specify resource-based permissions, you can attach a policy to the resource, such as an Amazon SNS topic, an Amazon S3 bucket, or an Amazon Glacier vault. In that case, the policy has to include information about who is allowed to access the resource, known as the principal. (For user-based policies, the principal is the IAM user that the policy is attached to, or the user who gets the policy from a group.)

The following example shows a policy that might be attached to an Amazon S3 bucket and that grants permission to a specific AWS account to perform any Amazon S3 actions in mybucket. This includes both working with the bucket and with the objects in it. (Because the policy grants trust only to the account, individual users in the account must still be granted permissions for the specified Amazon S3 actions.)

{
  "Version": "2012-10-17",
  "Id": "S3-Account-Permissions",
  "Statement": [{
    "Sid": "1",
    "Effect": "Allow",
    "Principal": {"AWS": ["arn:aws:iam::ACCOUNT-ID-WITHOUT-HYPHENS:root"]},
    "Action": "s3:*",
    "Resource": [
      "arn:aws:s3:::mybucket",
      "arn:aws:s3:::mybucket/*"
    ]
  }]
}

IAM policies control access regardless of the interface. For example, you could provide a user with a password to access the AWS Management Console, and the policies for that user (or any groups the user belongs to) would control what the user can do in the AWS Management Console. Or, you could provide the user with AWS access keys for making API calls to AWS, and the policies would control what actions the user could call through a library or client that uses those access keys for authentication.

For basic example policies that cover common scenarios, see Example Policies for Administering AWS Resources, AWS Services That Work with IAM, and the AWS Policy Examples page in the AWS Sample Code & Libraries section of the AWS website.

AWS managed policies and the Policy Generator are available from the IAM console in the AWS Management Console. For more information about creating policies in the console, see Working with Policies. Also, you can use the AWS Policy Generator online to create policies for AWS products without accessing the console.

Multiple Statements and Multiple Policies

You can attach more than one policy to an entity. If you have multiple permissions to grant to an entity, you can put them in separate policies, or you can put them all in one policy.

Generally, each statement in a policy includes information about a single permission. If your policy includes multiple statements, a logical OR is applied across the statements at evaluation time. Similarly, if multiple policies are applicable to a request, a logical OR is applied across the policies at evaluation time.

Users often have multiple policies that apply to them (but aren't necessarily attached to them). For example, IAM user Bob could have policies attached to him, and other policies attached to the groups he's in. In addition, he might be accessing an Amazon S3 bucket that has its own bucket policy (resource-based policy). All applicable policies are evaluated and the result is always that access is either granted or denied. For more information about the evaluation logic we use, see IAM Policy Evaluation Logic.

Policy Structure

Each policy is a JSON document. As illustrated in the following figure, a policy includes:

Each statement includes the core information about a single permission. If a policy includes multiple statements, AWS applies a logical OR across the statements at evaluation time. If multiple policies are applicable to a request, AWS applies a logical OR across the policies at evaluation time.

The information in a statement is contained within a series of elements. For information about these elements, see IAM Policy Elements Reference.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "FirstStatement",
      "Effect": "Allow",
      "Action": ["iam:ChangePassword"],
      "Resource": "*"
    },
    {
      "Sid": "SecondStatement",
      "Effect": "Allow",
      "Action": "s3:ListAllMyBuckets",
      "Resource": "*"
    },
    {
      "Sid": "ThirdStatement",
      "Effect": "Allow",
      "Action": [
        "s3:List*",
        "s3:Get*"
      ],
      "Resource": [
        "arn:aws:s3:::confidential-data",
        "arn:aws:s3:::confidential-data/*"
      ],
      "Condition": {"Bool": {"aws:MultiFactorAuthPresent": "true"}}
    }
  ]
}